The invention relates to a process for separating ethylene and ethane from a hydrocarbon steam-cracking effluent that contains in particular ethane, ethylene and acetylenic compounds.
The production of ethylene and propene by steam-cracking of hydrocarbons uses processes that make it possible to separate the ethylene and the propene of lighter gases that are contained in the effluents of cracked gases. Crude ethylene and propene (C2/C3 fractions) also contain undesirable acetylenic compounds that should be recovered. When these compounds are desired as co-products, they can be extracted by a solvent. Such processes are, however, very dangerous because of the instability of highly concentrated acetylenic compounds.
The prior art is illustrated by Patent Applications U.S. Pat. No. 3,755,488, EP-A-0 825 245 and WO-93 24428.
The acetylenic compounds conventionally are converted into ethylene and propene by hydrogenation. A process for separating ethylene from methane via at least one distillation column (demethanizer) whose top fraction is condensed at a very low temperature by the ethylene is known by Patent U.S. Pat. No. 4,900,347.
These condensation conditions require the use of stainless steel material and consume a lot of energy.
A process of another type (ALCET, registered trademark) that is less expensive was described by LAM, W. K., AICHE Spring National Meeting April 1986, New Orleans. It comprises, in a series, a distillation stage (deethanizer, in English, to draw off C3+ hydrocarbons at the bottom of the column, or depropanizer, in English, to draw off C4+ hydrocarbons at the bottom of the column), a compression stage of the top gaseous fraction, a stage for hydrogenation of this gaseous fraction, a stage for separating a gaseous phase that is introduced into a solvent absorption column, and a liquid phase that is recycled as reflux. At the top, the absorption column delivers a light phase that contains hydrogen and methane that is separated by condensation with propane and/or propene, and at the bottom, the column delivers a solvent phase that contains the desired C2 compounds. This solvent phase is then regenerated, the solvent is recycled in the absorption column, and the desired C2 compounds are recovered as feedstock of a subsequent downstream treatment, of polymerization, for example.
In such an ALCET process, the stages of compression and heating as well as the stage for hydrogenation of the acetylenic compounds take place in the presence of the entire top gaseous fraction that contains in particular hydrogen, carbon monoxide and methane. This involves larger-size equipment and larger investments. In addition, the reaction heat that is involved in the hydrogenation reactor and the fact of operating in vapor phase with excess hydrogen ensures that the temperature of the reactor has a tendency to increase, which can impair the selectivity of the hydrogenation reaction of the acetylenic compounds, whereby the ethylene can be partly hydrogenated in turn. To eliminate this, the ALCET process is carried out in the presence of two hydrogenation reactors with intermediate cooling.
These hydrogenation reactions can be accompanied by the formation of polymers (green oil) that gradually foul and deactivate the catalyst. Because the reactions are carried out in gaseous phase, these compounds cannot be washed and eliminated at least in part.
Finally, the presence of CO and H2 can result in the formation of methane and water that it is necessary to eliminate in the downstream condensation treatment.
One of the objects of the invention is to eliminate the drawbacks of the prior art, in particular to obtain a mixture that contains at least 85% by weight of ethylene and that can be used directly for the synthesis of polyethylene and plastics.
Another object is to carry out at least in part a hydrogenation in liquid phase, which is very selective and which essentially eliminates all of the triple-bond compounds and the diene compounds.
It was noted that by first carrying out a stage where a steam-cracking effluent is absorbed by a solvent and in particular the one that is obtained from a furnace, for example, a ceramic furnace that operates at a very high temperature, then a hydrogenation stage in mixed liquid phase and vapor phase of the effluent at the bottom of the absorber and finally stages for separating effluents that are produced and that comprise a stage for regenerating solvent that do not use cryogenic condensations, a final product of ethylene and ethane of excellent quality was obtained at a reduced cost.
More specifically, the invention relates to a process for separating a mixture that consists essentially of ethane and ethylene from a hydrocarbon steam-cracking effluent, whereby the effluent comprises hydrogen, methane, ethylene, ethane, acetylene, methylacetylene, propadiene, propene and hydrocarbons with at least 4 carbon atoms. The process is characterized in that:
Said feedstock (1) is absorbed in at least one absorption column (7) by a cooled solvent phase (9) under suitable absorption conditions, and a gaseous phase (50) that contains in particular hydrogen and methane at the top of the column and a partly liquid phase at the bottom of the column that contains the solvent that is enriched with ethylene, ethane, acetylene, methylacetylene, propadiene, propene and hydrocarbons with at least 4 carbon atoms are recovered;
the liquid phase is hydrogenated in at least one catalytic hydrogenation zone (15) in the presence of hydrogen and a hydrogenation catalyst under suitable hydrogenation conditions, and at least one liquid phase that is at least partly hydrogenated and that essentially does not contain acetylene is recovered.
The following stage sequence is carried out:
a) Said liquid phase that is at least in part hydrogenated is circulated in at least a first distillation column (16), and there is recovered: at the top of the column, a gaseous phase that is condensed to separate a non-condensed vapor phase and a liquid phase that contains hydrocarbons with two carbon atoms as reflux; by a lateral draw-off, said mixture (17) that consists essentially of ethane and ethylene; and at the bottom of the column, a hydrocarbon-enriched solvent phase (19) with at least 3 carbon atoms (C3+)
b) The solvent phase is regenerated in at least a second distillation (regeneration) column (22), and a C3+-enriched fraction is recovered, and at the bottom, a regenerated solvent phase is recovered;
The solvent phase is cooled, and it is at least partly recycled in the absorption column.
By carrying out the hydrogenation of a partly liquid phase that contains many fewer light compounds (H2, CH4) than the hydrogenation feedstock according to the ALCET process upstream from the solvent absorption stage, the temperature of the exothermic reaction that, moreover, is carried out toward 80xc2x0 C. is monitored much better. In addition, a much more selective reaction is obtained, without loss of ethylene, in a reactor of smaller size and with a catalyst whose service life is increased because the polymeric compounds are washed by the liquid phase and eliminated by a downstream purge.
According to a characteristic of the process, the gaseous phase that contains in particular methane and hydrogen, obtained from the absorption column, is condensed at least in part to deliver a liquid phase (52) that is recycled at least in part as reflux in the column, and a vapor phase (11) that is high in methane and hydrogen.
This reflux can contain the cooled solvent that supplies said column when supply of solvent of the column is connected to the output of the gaseous phase that is to be condensed and not directly to the top of the absorption column. The solvent is generally cooled between xe2x88x9210xc2x0 C. and xe2x88x9260xc2x0 C. before it enters the column.
According to another characteristic of the process, the liquid phase at the bottom of the absorption column can be reheated by heat exchange with the regenerated solvent phase that is obtained from the regeneration distillation column.
According to another characteristic of the process, the gaseous phase that is obtained from the first distillation column can be condensed by propane, propene or a mixture of the two.
It may be advantageous, according to a first variant, that at least a portion of pressurized vapor phase (11) that is obtained from the absorption column and that results from the condensation stage is reheated and then is separated on a suitable separation membrane (33), and a hydrogen- and methane-enriched retenate (35) and a permeate (34) that contains solvent, ethylene and ethane, which is compressed and recycled in said first distillation column, are recovered. It may be advantageous to reduce the pressure on the retentate in a turbo-pressure regulator.
According to a second variant, at least a portion of the vapor phase that is obtained from the absorption column can be reduced in pressure in a turbo-pressure regulator, and a liquid phase is recovered that is reheated and that is sent as reflux into first distillation column (16).
According to another characteristic of the invention, the C3+-enriched fraction that is obtained from the second distillation-regeneration column can be condensed at ambient temperature, preferably by water, to deliver a C3+ fraction that is recycled in part as reflux in the second distillation (regeneration) column and in part in a column for washing the steam-cracking effluent.
According to a first embodiment that proves very economical when the steam-cracking effluent is obtained from a heavy feedstock (naphtha, for example), this effluent contains heavier C4+ hydrocarbons that can be washed in a plate or packing washing column by the recycled C3+ fraction obtained from the second regeneration distillation column, and a fraction is recovered from top to bottom that is cooled and that is sent into the absorption column and a bottom fraction is recovered that contains the heaviest hydrocarbons.
According to another method that can be applied to steam-cracking effluents that also contain C4+ hydrocarbons, it may be preferable to carry out, upstream from the absorption column, a distillation stage (depropanizer) that delivers at the bottom of the column a heavy C4+ fraction that is recovered and at the top a lighter gaseous fraction that is compressed at least once, cooled and condensed. A liquid phase is then obtained that is recycled as reflux in the column relative to said distillation stage and a gaseous phase that is introduced into the absorption column and that treats only hydrocarbons with at most three carbon atoms.
The conditions of the solvent absorption stage can be as follows:
the ratio of solvent to feedstock in the absorption column is between 0.3 and 2 and preferably between 0.5 and 1,
the temperature at the top of the column is xe2x88x9210xc2x0 C. to xe2x88x9260xc2x0 C., preferably between xe2x88x9235xc2x0 C. and xe2x88x9245xc2x0 C.,
the pressure is between 10 and 50 bar, and preferably between 25 and 35 bar (1 bar=10xe2x88x921 Mpa),
number of theoretical stages: 15 to 40.
The temperature in the condensation flask by the propane and/or propene of the gaseous fraction of the top of the absorber is generally between xe2x88x9210xc2x0 C. and xe2x88x9260xc2x0 C. under 10 to 50 bar and preferably between xe2x88x9235 and xe2x88x9245xc2x0 C. under 25 to 35 bar.
The solvent is usually selected from the group that is formed by toluene, pentane, hexane, the toluene-benzene mixture and the cyclohexane-toluene mixture, but any other solvent that can absorb the C2+ hydrocarbons and that is stable in temperature can fall within the scope of the invention.
The conditions of the hydrogenation stage can be as follows:
temperature 10-150xc2x0 C., preferably 60-100xc2x0 C.,
volumetric flow rate (LHSV) (hxe2x88x921): volume/volume x hxe2x88x921: 5-50 and preferably 10-30
pressure 10 to 30 bar, preferably 15-25 bar,
catalyst: with a palladium base and optionally at least one metal of group IB, preferably Ag or Cu
H2: 0 to 10% in addition relative to the stoichiometry,
H2 purity: at least 80 mol %, preferably 90 to 99%.
The preferred substrate of the catalyst can be an alumina with a small specific surface area.
The bottom temperature of the first distillation column is usually between 80 and 300xc2x0 C. and varies based on the selected solvent. For example, when the solvent is toluene, the bottom temperature can be 150 to 300xc2x0 C. and preferably between 180 and 240xc2x0 C. When pentane is selected, the bottom temperature can be between 80 and 160xc2x0 C., preferably between 100 and 140xc2x0 C. The top pressure of the column can vary between, for example, 10 and 25 bar and preferably between 15 and 18 bar.
The temperature and the pressure in the condensation flask by the propane-propene mixture of the top fraction of the first column is generally xe2x88x9210 to xe2x88x9260xc2x0 C. under a pressure of 10 to 20 bar and preferably xe2x88x9235 to xe2x88x9245xc2x0 C. under 12 to 17 bar.
By contrast, the second distillation column can be operated at a column bottom temperature of 80xc2x0 C. to 250xc2x0 C. and under a top pressure of 5 to 15 bar. The temperature in the condensation flask of the gaseous fraction by water is 10 to 50xc2x0 C. under 5 to 15 bar and preferably 30 to 40xc2x0 C. under 6 to 8 bar, which is particularly economical.
To ensure the conversion of the acetylenic compounds and propadiene, it may be advantageous that the hydrocarbon-enriched solvent phase that is obtained from the first distillation column be hydrogenated in a second catalytic hydrogenation zone (20) in the presence of hydrogen, and the hydrogenation effluent is introduced into the second distillation column, from which a portion of the C3+ fraction is recovered in the upper portion of said column by a lateral draw-off.
According to a more advantageous variant that makes it possible to carry out the essentially total hydrogenation of methyl-acetylene and propadiene, it is possible to draw off laterally from first distillation column (16), below the point of introduction of liquid phase (15a), hydrogenated in said first column, a partly liquid stream that is reheated and hydrogenated in a second catalytic hydrogenation zone (20) in the presence of hydrogen, and a hydrogenated stream is recycled at an intermediate point of said first column between the lateral draw-off point and the point of introduction of hydrogenated liquid phase (15a).
It is thus possible to select the appropriate level of temperature of the hydrogenation reaction. In addition, all of the excess hydrogen that is brought to the hydrogenation reactions is found only in the first distillation column. Furthermore, all of the heat that is released by the hydrogenation reaction is provided to said first column, the only one to which the reboiling heat is supplied. Finally, pasteurization in the second distillation column and therefore a lateral draw-off line of the C3+ fraction of this column are avoided.
The invention also relates to a device for separation of hydrocarbons with two carbon atoms comprising a solvent absorption column (7) that has a solvent feed at the top of the column, an inlet (1) for a hydrocarbon feedstock, an outlet (50) at the top of the column for a gaseous phase that comprises cooling means (51), condensation means (10) that have an outlet (11) for a first gas and an outlet (52) for a condensed liquid phase and reflux means of said condensed liquid phase in the absorption column, and an outlet (12) at the bottom of the column for a solvent liquid phase. It also comprises:
at least one catalytic hydrogenation reactor that has an inlet connected to the outlet at the bottom of the solvent liquid phase, comprising a hydrogen feed and an outlet (15a) for a hydrogenation effluent;
a first distillation column (16) that has an inlet that is connected to the outlet of the hydrogenation effluent, a first outlet (53) above a gaseous phase that comprises cooling means (54), condensation means (55) that have an outlet (18) for a second gas and an outlet for a condensed liquid phase, and reflux means of the condensed liquid phase in the first distillation column, a second outlet (17) that delivers hydrocarbons with two carbon atoms, and a third lower outlet (19) that delivers a hydrocarbon solvent liquid phase;
a second distillation column (22) that has an inlet connected to the third outlet of the first column, a first outlet for a gaseous phase that comprises cooling means at ambient temperature, condensation means that have a gas outlet and an outlet (58) for a condensed liquid phase and reflux means of a portion of the condensed liquid phase in the second distillation column; a second upper outlet that delivers hydrocarbons and a third lower outlet (26) that delivers the solvent; and
means (61, 9) for recycling the solvent that comprise cooling means (8, 13) that are connected to the top of the absorption column and to the third lower outlet of the second distillation column.
The invention will be better understood based on the figure and the example, which illustrate an embodiment that comprises in series an absorber of the steam-cracking effluent in the presence of a solvent, a hydrogenation reactor in liquid phase, a first and a second column for distillation-regeneration of the solvent and the recycling of the solvent to the absorber.
A 97% ethane feedstock, for example, is steam-cracked in a furnace under very severe conditions making it possible to obtain a steam-cracking effluent 1 that is dehydrated and compressed by means that are not shown in the figure. This effluent has a composition that is provided in the example.
This effluent is sent into the lower portion of a washing column 2 and is brought into contact in countercurrent with a liquid recycling stream 3 of a C3+ distillate (C3-C4) that is obtained from a regeneration distillation column 22 that is described below, according to a molar ratio of effluent/C3+ flow rates of between 0.01 and 0.10. At the bottom of the flask, the heaviest C4+ hydrocarbons that contain about 0.5% by weight of ethylene are recovered via a line 4.
A top effluent 5 of column 2 is cooled in a heat exchanger 6 and is introduced into an absorption column 7 that is often called xe2x80x9cdemethanizerxe2x80x9d in English. A stream for recycling a solvent, the toluene that was previously cooled, is mixed with a gaseous phase that is collected via a line 50 at the top of column 7.
The gaseous phase that contains the solvent and in particular hydrogen and methane is cooled (51) by propene to xe2x88x9240xc2x0 C. and condensed in a condensation flask (10) from which is drawn off a C2+-enriched liquid phase that contains the solvent that is sent as reflux 52 into the upper portion of the absorption column and a vapor phase 11 that is high in methane and hydrogen that can be separated later. Said reflux is brought into contact in countercurrent with the absorption feedstock. At the bottom of the absorption column, a partly liquid phase 12 that contains toluene that is enriched with ethylene, ethane, acetylene, methylacetylene, propadiene, propene and hydrocarbons with at least 4 carbon atoms is drawn off. This liquid phase is reheated in a heat exchanger 13 and sent into a catalytic hydrogenation reactor 15 in the presence of hydrogen that is introduced via a line 14 into line 12.
Essentially all of the acetylene and the propadiene are generally converted into ethylene and propene respectively. The majority of the methylacetylene is also converted. The increase of temperature because of the exothermic reaction in liquid phase generally does not exceed about 10 degrees. The hydrogenation effluent that is drawn off at the top of the hydrogenation reactor via a line 15a is sent into a first distillation column 16 that is called xe2x80x9cdeethanizerxe2x80x9d in English. At the top of the column, a gaseous phase is recovered via a line 53 that is cooled (54) to approximately xe2x88x9240xc2x0 C. by propene and condensed in a condensation flask 55, from which is separated a vapor phase 18 that contains hydrogen and in particular the excess hydrogen from the hydrogenation reactor and the ethylene that is picked up at the top. The separated liquid phase in flask 55 that contains the C2+ fraction is sent as reflux to the top of first column 16. From the latter, an ethylene-ethane mixture with more than 85% by weight of ethylene and less than 1 ppm (mol) of carbon monoxide and acetylene, for example, is drawn off laterally at about three plates below.
Non-condensed vapor phase 18 of condensation flask 55 can be recompressed via a compressor 31 and recycled under pressure under the reflux line of absorber 7 via a line 32.
Vapor phase 11 under pressure can be reheated by indirect exchange with the feedstock of the absorber thanks to exchanger 6 and sent into a membrane separator 33, from which is recovered a permeate 34 that essentially contains the heaviest C2+ hydrocarbons that are recycled once compressed by a compressor 37 in distillation column 16 via a line 37a and a retentate 35 that contains hydrogen, methane and carbon monoxide. The pressure of this pressurized retentate can be reduced in a turbo-pressure regulator 36 whose line recovers the gas. This turbo-pressure regulator puts compressor 31 into motion.
At the bottom of the first distillation column, a mixture of hydrocarbon-enriched solvent with at least 3 carbon atoms is recovered via a line 19. According to a first variant, if this mixture also contains methylacetylene and/or propadiene, the latter can be converted into propene in a second catalytic hydrogenation reactor 20 in the presence of hydrogen that is introduced via a line 14b at the top of the reactor. A second hydrogenation effluent is collected via a line 21 that laterally feeds a second distillation-regeneration solvent column that is called xe2x80x9cdebutanizerxe2x80x9d in English, approximately in the middle of the latter. At the top is recovered a C3+-enriched fraction that is cooled (56) to 30-35xc2x0 C. by water and that is condensed in a condensation flask 57. At the top of the flask, a light gas fraction is recovered via a line 25, and at the bottom, a C3+ liquid phase is recovered via a line 58, which is partly recycled as reflux 60 and partly recycled via a pump 59 and line 3 as reflux in washing column 2.
Furthermore, an effluent that contains C3+ hydrocarbons is drawn off laterally from the second distillation column via a line 24 that can be connected to line 4 from the bottom of washing column 2 for a treatment downstream from fraction C3-C4-C5.
According to a variant that is illustrated by the figure, a second hydrogenation reaction can be carried out in the following way, when the main hydrogenation reaction of the acetylenic or dienic compounds in reactor 15 is not complete.
Starting from the first distillation column, a lateral draw-off of a liquid stream, several plates below introduction line 15a, is sent via a line 73 through a heat exchanger 27. It is reheated indirectly via line 26 of the solvent, then introduced into a second catalytic hydrogenation reactor 75 in the presence of hydrogen that is provided via a line 14c that operates at a temperature that is generally less than 150xc2x0 C. The second hydrogenation effluent is reintroduced via a line 74 at a point of distillation column 16 that is located between line 15a for introduction of the distillation feedstock and draw-off line 73. By this arrangement, pasteurization in second distillation column 22 and lateral draw-off line 24 of the C3+ fraction that can be connected directly to line 60 is avoided.
Finally, via a line 26, regenerated solvent is collected from the bottom of the column that is recycled under pressure via a pump 61 and line 9 at the top of absorption column 7 in line 50 after a series of heat exchanges, in particular with a lateral reboiler 27 of the first distillation column, a preheating exchanger 13 of the hydrogenation feedstock, a reboiler of absorption column 7 and an exchanger 28, intended to cool the solvent.
A portion of flow 26 is also recycled as reboiling fluid in second distillation column 22 after a heat exchange 23 with effluent line 21 of the second hydrogenation reactor.
The accumulated polymers can be separated continually from the solvent by a suitable distillation of a minor portion of the regenerated solvent flow that is drawn off via a line 29. It is reintroduced after distillation via a line 30 in line 9 upstream from cooling exchanger 28.